Effects of chewing gum for tear production in healthy young subjects

Chewing is not simply a voluntary movement of the teeth and jaws but involves a high-level integration of movements controlled by the brain (Momose et al. 1997). Chewing is also known to act on the sympathetic and parasympathetic nerves (Kanno et al. 2019). Our prior study showed that chewing gum affects the parasympathetic nerves predominantly acting to contract the iris sphincter muscle and the circular Müller’s fibres of the ciliary muscle in healthy young subjects (Asakawa et al. 2020). Because tear production by the lacrimal glands is stimulated by the parasympathetic nerves (Dartt 2009), if chewing gum results in parasympathetic nerve predominance, tear production may also be improved. However, it is still unclear how chewing gum involves the lacrimal glands. We, therefore, investigated the effects of chewing gum for tear production in healthy young subjects. An open-label randomized crossover trial was conducted (approval No. 2020-035). This study followed the tenets of the Declaration of Helsinki for research involving human subjects, and informed consent was obtained. Four subjects (severe eye dryness, excessive blinking and two disliked chewing gum) were excluded. Thus, we examined 24 eyes of 12 healthy young subjects (six men, six women) (age, 20–23 years). Schirmer I test (White Medical Co., Ltd., Tokyo, Japan) with topical anaesthesia (0.4% oxybuprocaine hydrochloride, Santen Pharmaceutical Co., Osaka, Japan) was performed in the same order for each subject for each eye, right eye first. Thirty seconds after instillation of eye drop, the test was performed for 5 min with or without chewing two pieces of gum for 5 min. Subjects chewed gum on Day 1 and did not chew gum on Day 2, and vice versa. The test starting times, food and beverage intake (water ad libitum), were matched on 2 days. The comparisons of values were performed using the Wilcoxon signed-rank test. Effect size (r) was also calculated (SPSS, version 20.0; IBM Corporation, Armonk, NY, USA). Results are presented as median and interquartile range (25th, 75th percentiles). P values of < 0.05 were considered statistically significant. For subjects with a result of ≥ 35 mm within 5 min, data analysis was 35 mm. The values of the Schirmer I test were 22 (14, 35) and 17 (12, 26) mm with and without chewing gum, respectively. These were significantly longer with chewing gum (P = 0.015, r = 0.489) (Table 1). Chewing is not only a physiological action for nutrient intake through the ingestion and digestion of food, but it is also regulated by complex functions of the nerves, oral organs and tissues. The effects of chewing gum demonstrated that the maintenance of homeostasis, in the forms of blood pressure and heart rate (Kanno et al. 2019), and also actions on the eye (Asakawa et al. 2020), is mediated by the sympathetic and parasympathetic nerves. In the present study, we found that chewing gum may help activate the lacrimal glands by the action of the parasympathetic nerves. Tears are also derived from the meibomian glands, and meibomian secretions help stabilize the tear film (Arita et al. 2017). In our prior study, there were no significant differences after chewing gum in the ring break-up time values using the RT-7000 and Tear Stability Analysis System (Tomey Co., Nagoya, Japan) (Asakawa et al. 2020). One of the positive aspects is that chewing gum may help activate the lacrimal glands but not the meibomian glands. In conclusion, chewing gum may help activate the lacrimal glands, conceivably helping to alleviate eye dryness. However, the clinical implications of these findings remain unclear. We will subsequently conduct a similar study to elucidate the effects of chewing gum to alleviate eye dryness.